High voltage capacitor route with integrated failure point

ABSTRACT

An implantable medical device may have a circuit failure mode. The disclosed circuit may have an integrated failure point designed to fail prior to those portions of the circuit. The integrated failure point may include a narrowed portion of a high voltage lead and a grounded lead having a narrow gap separating the grounded lead from the narrowed portion of the high voltage lead. During a high stress fault condition the narrowed portion of the high voltage lead acts as a fuse, forming a vaporized cloud of metal, which shorts current in the high voltage lead across the narrow gap to the grounded lead, thus protecting the remaining portion of the circuit from the high stress condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. application Ser. No. 11/677,793,filed on Feb. 22, 2007, the benefit of priority of which is claimedherein, and which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to fabrication of an integratedfailure point in a circuit design to protect the remainder of thecircuit from damage, and to device electrical overstress protectionmethods, and to implantable medical devices.

BACKGROUND

As implantable medical devices continue to decrease in size, the circuitand component layouts enclosed within the device will generally increasein density. This increase in device density may result in device spacingand circuit metal line spacing that may be very close, and may result incross talk or even arcing. This may be an issue, particularly in anelectronic device using high voltage levels.

OVERVIEW

The present inventors have recognized that, in the case of implantablecardioverter defibrillators (ICDs) and cardiac resynchronization therapydefibrillators (CRT-Ds), close circuit trace spacing may lead to asituation where an electrical overstress on one circuit may damageadjacent circuits. This is because the electrical overstress may causeintense local heating, and circuit burnout that can overheat or short toadjacent circuits.

Electronic devices may use fuses to burn out and open up a metal tracethat is carrying more power than the downstream components canwithstand. A potential issue with the use of fuse, however, is that thetime during which the fuse is burning out may result in significantdamage to the remainder of the circuit downstream from the fuse.Essentially, the fuse does not burnout fast enough to fully protect therest of the circuit.

Thus, there is a need for improved structures and methods with respectto the manufacture of implantable medical devices. In particular, thereis a need for a power disconnect method that is faster than a fuse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a circuit;

FIG. 2 illustrates an integrated protection circuit according to variousexamples; and

FIG. 3 illustrates an implantable device in accordance with variousexamples.

DETAILED DESCRIPTION

In any electronic device, and especially implantable medical devices,there is a need to disconnect circuit elements and conductive lines froma failed portion of the circuit, to prevent further circuit damage. Asimple fuse may serve the function of disconnecting the circuit from anoverstress situation, but may not burn out fast enough to preventdownstream damage. This may be true for various reasons, including notproviding the fuse with sufficient resistance to rapidly heat during anoverstress situation, or it may be due to evaporation of the fusematerial during burn out forming a conductive plasma around the burn outsite. This arcing may result in the fuse remaining in a conductive statefor a short period after the evaporation of the fuse, at least until theevaporated fuse material disperses enough to prevent the arcing over thefuse gap. Thus, the gap formed by the rapid heating of the resistivefuse element may not be enough to fully protect the rest of the circuit,or the patient. One way to increase the rate of disconnecting a circuitfrom an overstress situation, either high voltage, high current or highpower, is to add a switch responsive to the overstress situation,wherein the switch at least temporarily connects the overstress to asink, or a ground, or negative pole of a capacitor.

Another way to improve the disconnect rate would include buildingintentional failure points into the circuit, for example, at locationsthat have lower resistive paths to a ground supply. Integrating thesefailure points into the circuit design may increase the ability of thedevice to rapidly and reliably disconnect the circuit from anyoverstress surges, particularly in view of the repeatability andprecision of integrated manufacturing operations, such as PCB and ICmanufacturing. In an illustrative example, an integrated failure pointmay include a fuse formed by narrowing a conductive trace to increasethe resistance to a level that will burn out by evaporation of theconductive trace material at an overstress level that is below a valuelikely to cause damage to the remainder of the circuit or the patient. Asecond conductive trace, connected to a ground voltage source by a lowresistance connection, may dead-end in close proximity the fuse, such asnear the high voltage end of the fuse. In certain examples, the secondconductive trace is separated from the fuse by a spacing that is equalto the design rule specifying the minimum metal-to-metal spacing of thecircuit technology being used to fabricate the circuit. For example, astandard PCB manufacturing process may have copper and solder conductivetraces. A minimum spacing design rule may specify that such traces mustbe separated from each other by about 0.010 inches. During an overstresssituation, the fuse may begin to overheat and vaporize, thereby forminga gap having a length that is larger than the minimum design rulespacing. Thus, the dead-end of the second conductive trace may now becloser to the high voltage end of the fuse than the other side of theburned-out gap in the fuse. In addition, since the second conductivetrace is directly connected to ground, the current path through thesecond conductive trace has a lower resistance compared to the currentpath across the fuse gap and through the remainder of the circuit. Inthis situation, the vaporized fuse material may form a temporaryconductive arc between the high voltage end of the fuse and the secondconductive trace. This will rapidly reduce the current flowing to theremainder of the circuit. The vaporized fuse material supporting thetemporary arc forms a conductive gas, which may be called a plasma, andthe described arrangement may be called a plasma switch.

It should be noted that the described illustrative examples are notintended to be limited to the disclosed arrangements and methods, butmay include any method of forming a circuit, fuse and switch. Forexample, the structure may be formed of any combination of metals, PCBs,hybrids or semiconductors, having insulator oxides, nitrides, polymersor combinations thereof. In another example, the structure may be formedusing the circuit and fuse with no switch where the fuse has beenpositioned to minimize damage to adjacent circuits.

FIG. 1 illustrates one example of a circuit for protecting againstoverstress situations. The device 100, which may comprise a hybridcircuit, devices on a PCB, or a monolithic IC, has a circuit 102 to beprotected from an overstress generated by a failure or power spike in apower source 104, such as a high voltage capacitor. The circuit 102 andthe power source 104 are connected by conductive traces 106, and a fuse108. The fuse 108 may have a higher resistance that the conductivetraces 106, and be formed of a different material, or be formed of thesame material but having a smaller current carrying cross section. Onemethod of reducing the current carrying cross section is to reduce theconductive trace width to the minimum design rule for the technology.The fuse 108 will evaporate under the heating effects of an overstresssituation, for example at the location 110, which may be at theconnection point between the fuse 108 and the conductive trace 106closest to the power source 104. The evaporation of the fuse at location110 creates a gap in the fuse 108, but during the evaporation process,the gaseous fuse material may form a plasma in the region 110 andtemporarily continue to conduct current and voltage across the gap infuse 108. The addition of a switch connecting the region 110 to thenegative portion of the power source 104 could be implemented to providea lower resistance than the path through the conductive traces 106 tothe functional circuit 102 and back to the power source 102. In such ascenario, the switch would essentially short circuit the functionalcircuit 102, and take it out of the power circuit, thus providingimproved protection. The present inventors have recognized that a plasmaswitch is inexpensive to form, and may provide a reliable andreproducible failure point that may be integrated into the circuit, suchas during a standard circuit manufacturing process.

FIG. 2 illustrates an example of an integrated protection circuit 200that includes a plasma switch formed by the addition of the conductivetrace 212 to the circuit of FIG. 1. In this arrangement the evaporationof the fuse material in the region 210 allows any potential arcing tojump to the lower resistance path formed by the proximity of theconductive trace 212 and the lower resistance path formed by traces 212and 206, as compared to the longer path through the functional circuit202. Such plasma switches may be particularly useful in high voltage orother medical devices, integrated circuits, and other electronicdevices. In an example, the conductive traces 212 and 206 are bothformed in a single metal trace layer of a multilayer PCB, and are thusformed of the same material having the same thickness and insulationproperties. In such an example, the trace 212 will be separated from thefuse 208 by at least distance that is equivalent to the minimum designrule spacing between such same-layer conductive traces. The overstresslevel that is capable of forming the plasma switch depends upon designfactors including the material used and the spacing. In certainexamples, smaller spacing (and consequent lower voltage plasma switchoperation levels) may be obtained by using different conductor levelsfor traces 212 and 206, and using the layer-to-layer alignment accuracyof the manufacturing process to determine the gap, particularly usefulif such accuracy is reproducible in a manufacturing setting. In certainexamples, the region over at least a portion of the fuse 208 and the tipof the trace 212 may have an overlying insulator layer removed, such asto increase the rate at which the plasma switch forms, particularlyuseful if long term corrosion resistance of the fuse is not compromised.

FIG. 3 illustrates an implantable device 300, such as a defibrillator orother cardiac function management device, placed within the body of apatient. The defibrillator circuit may be contained within a hermetichousing body 302, which may be formed of a biocompatible material. Thefunctional circuit 304 may contain the measurement and evaluationcircuits to determine if the patient requires shock therapy, which maybe delivered to the heart of the patient by electrodes, which may belocated on one or more therapeutic leads traveling out of the pulsegenerator body 302 through the header 306 via one or more hermetic passthrough 308. The circuit 304 may connect a power source 310, for examplea high voltage capacitor, to the heart for shock therapy, such as pacemaking or cardioverting. A plasma switch located between the powersource 310 and the functional circuit 304 may be used to disconnect thepower source 310 from the functional circuit 304 in the case of anelectrical overstress situation, which may otherwise cause anundesirable high voltage current to pass into the patient's heart orcause damage to unrelated portions of the device that might otherwisestill provide therapy or failure enunciation.

It should be understood that some examples are equally applicable to anysize and type of circuit and are not intended to be limited to aparticular type of device, such as the illustrative implantable cardiacfunction management device. For example, the described examples may beformed on an integrated circuit (IC), on a thick film hybrid circuitboard, or on a printed circuit board (PCB), each with different minimumdesign rule line width and spacing for the fuses, switches and devices.Another example may be to position the fuse internal to the power source310 or in the header 306.

Some examples may include an implantable medical device including avoltage source having first and second terminals, and a functionalcircuit, also having first and second terminals coupled to the first andsecond terminals of the voltage source, for example power and groundterminals. There may be a fuse coupled between one of the first andsecond terminals of the voltage source and one of the first and secondterminals of the functional circuit to protect the functional circuitfrom electrical overstress situations, and thus protect a patient frominadvertent harm. The fuse may be located either inside or outside ofthe voltage source, and may be coupled in series with the first andsecond terminals. The fuse alone may not interrupt the electricaloverstress immediately due to vaporized portions of the fuse materialcontinuing to conduct high voltage or current flows across the burnedfuse for a short time period. To address this issue there may be aplasma switch, adapted such that when power in the fuse exceeds aselected fault power threshold, causes an arcing current to be conductedfrom the fuse to a node at the other one of the first and secondterminals of the voltage source such that the current is directed awayfrom the functional circuit. The plasma switch essentially forms animmediate shunt to direct power away from the functional circuit.

The plasma switch may include a shunting conductor with a first terminallocated at a specified distance away from the fuse, or at a specifieddistance away from a conductor to or from the fuse, and the conductormay include a second terminal coupled to the other one of the first andsecond terminals of the voltage source, for example power and ground.The plasma switch at the specified distance from the fuse or theconductor near the fuse may establish the arcing current to shunt thepower away from the functional circuit when the fault power threshold isexceeded. The specified distance between the shunting conductor and thefuse may typically be substantially equal to the minimum design ruledistance between unrelated conductors of the printed circuit board thatincludes the plasma switch and functional circuit, in order to createthe plasma switch arc at the lowest possible voltage level, unless themaximum desired voltage level is higher than the lowest obtainablelevel, in which case the spacing between the plasma switch conductor andthe fuse will be increased to obtain the desired voltage level for theplasma switch operation. The fuse and plasma switch may both be aportion of a single wiring layer of the printed circuit board so thatthe insulation between wiring layers does not interfere in the arcvoltage point, and the printed circuit board may include a hole in theboard insulation or dielectric in a region near the fuse and the plasmaswitch. The dielectric material may include polyimide, paralene or otherorganic insulative material. Selecting the fault power threshold totrigger the plasma switch may include determining a combination offeatures such as the wiring material, wiring thickness and width, thefuse width and ground conductor width for a wiring layer of the printedcircuit. The wiring material may include copper, tin or solder.

The apparatus may comprise a first conductor having a first resistanceper unit length connecting the first terminal to a resistive circuitelement having a resistance per unit length greater than the resistanceper unit length of the first conductor, so that the resistive element isthe fuse. A second conductor may connect the resistive circuit elementto the first terminal of the functional circuit, and a third conductormay connect the second terminal of the functional circuit to the secondterminal of the voltage source, thus completing the normal functionalcircuit power and ground connections. A fourth conductor may connect thesecond terminal of the voltage source, for example the ground terminal,to within a predetermined distance of the resistive circuit element,such as a specified minimum design rule spacing. Illustrative examplespacing for a printed circuit board may be about 0.010 inches, withsmaller possible spacing for hybrid circuits and for integrated circuits(ICs). This may allow the arcing current to flow between the resistiveelement and the fourth conductor when the fault power threshold isexceeded. The fourth conductor may thus provide an electrical shuntingconductor to pull power away from the functional circuit while the fuseburns completely open, and thus improve protection of the functionalcircuit over a simple fuse itself.

The resistive circuit element may be formed of a portion of the firstconductor, simply having a narrower width than other parts of the firstconductor, to form a resistor, or it may be formed of a material havinga higher resistivity. The voltage source may include various powersources including a capacitor for storing the voltage and current neededfor a therapeutic cardiac shock, such as may be needed to restart aheart, to stop fibrillations and restart a heart, or to cardiovert aheart either synchronously or asynchronously. Illustrative examples oflimits for a functional circuit may include high current capability inthe range of 10-20 amps, an overstress level in the range of 60-80 amps,and a voltage level of about 700 volts.

Another example may include a method for forming an implantable medicaldevice circuit on a printed wiring board, a hybrid circuit board, or onan integrated circuit device. The method may include forming a fusebetween a power source and the medical device circuit and forming aplasma switch between the fuse and a ground source. The plasma switchmay disconnect the power source from the medical device circuit and atleast temporarily connect the power source to the ground source inresponse to an electrical overstress condition. Forming the plasmaswitch may include locating a portion of an electrical conductor that isdirectly connected to the ground source, close to a portion of the fuse,or a portion of an electrical conductor between the power source and thefuse. The separation distance is generally selected to form an arc whenthe power source exceeds a threshold value. The arc provides a lowresistance path to ground during the fuse burnout, thus reducing oreliminating the current flow to the medical device circuit more quicklythan with a fuse alone. Triggering the plasma switch may includeselecting a combination of the distance between the electrical conductorconnected to the ground source and the portion of the fuse, the fusematerial, thickness, length, and width of the fuse, which partiallycontrol the fuse resistance, and a power source voltage level whichrepresents the electrical overstress condition. Forming the fuse mayfurther include selecting the fuse properties to cause the fuse tovaporize into a cloud of ions when the electrical overstress conditionoccurs. The cloud of ions form a temporary arc between the fuse and thenearby ground conductor to shunt the electrical overstress to ground andaway form the remainder of the circuit. In an example, the distancebetween the electrical conductor connected to the ground source and thefuse is selected to be substantially equal to the minimum design rulespacing for two unconnected metal features on the printed circuit board,hybrid circuit, or integrated circuit containing the plasma switch.

In another example, a method for delivering defibrillation shock energyfrom an implantable shock therapy energy storage capacitor to a subjectthrough a functional circuit may include detecting an electricaloverstress condition, and shunting the defibrillation shock energy fromthe capacitor away from the functional circuit and to a circuit power orground node in response to detecting the electrical overstresscondition. The shunting may be provided by a switch, a plurality ofresistors in a network, a fuse, a transistor, a Hall effect switch, amagnetic switch, a microswitch, a solenoid, or a plasma switch. Shuntingmay open the electrical connection to the functional circuit, and closea temporary electrical connection to the ground node, thus protecting apatient from the overstress.

Detecting the electrical overstress condition may be by selecting a fusematerial, a fuse material vaporization temperature, a fuse crosssectional area, a fuse length, a overlaying insulation material, and afuse resistance to cause the fuse to heat to the vaporizationtemperature at the electrical overstress condition. The fuse may thenvaporize and form an open conductor to the functional circuit, thuspreventing the circuit from being damaged by the overstress. Forming anelectrical gap between the fuse material and the electrical connectionto the ground node, where the gap is designed to have a dimensionselected to be within a cloud of vaporized fuse material caused by theelectrical overstress condition, may improve the disconnection speed.The cloud may more easily form an electrical conduction path to theground node, thus shunting current away from the functional circuitwhile the fuse is still burning open.

Another example may include an implantable defibrillator having animplantable bio-compatible hermetic casing, such as the pulse generatorscases which may be used in cardiac pacemakers. Within the casing theremay be a defibrillation circuit including one or more shock therapyenergy storage capacitors.

There may be at least one fuse located between the storage capacitor andthe defibrillation circuit, where the fuse may interrupt electricalpower to the defibrillation circuit in the case of an overstresscondition. The casing may further include a pacemaker, an acousticsensor, an RF generator, an antenna and a circuit for generating andreceiving RF signals from internal and external devices, a cardiacresynchronizer, or a cardioverter. The implantable defibrillator mayfurther include an electrical shunt switch connecting the fuse to aground connection. The electrical shunt switch may be a plasma switch, amechanical switch, an electronic switch or a magnetic switch.

An example of a method of protecting a circuit from electricaloverstress may include forming one or more fuses between a power supplyand the circuit to be protected, or in series connection with theterminals of the power supply, either inside or outside of the powersupply. There may be one or more grounded conductor lines located ashort selected distance away from each of the fuses to form integratedfailure points between the power supply and the circuit. The failurelevel may be selected by choosing the fuse material, thickness, width,length and resistance to vaporize the fuse material when the power levelreaches above a set point. The distance between one of the fuses and agrounded conductor may be smaller than one of the other distancesbetween a second fuse and a second grounded conductor to provide aseries of failure points that may be used to dissipate the stored energyin a distributed fashion. The functional circuit to be protected may bepart of a printed circuit board (PCB), and the resistors, fuses andswitches may be formed of the multiple conductor layers found inmultilayer PCBs, including copper traces and solder conductors. Theresistive element may be a fuse, and the fuse may include an opening inany overlaying insulator material to assist in adjusting the overstresslevel that will cause a blow out. In an example the fuse and thegrounded conductor may form a plasma switch that may rapidly temporarilyconnect the blowing fuse to the negative power terminal and to thuselectrically disconnect the remaining portion of the circuit from thepositive terminal during the period in which the fuse has vaporized, butthe metallic vapor is still able to conduct electrical overstress powerto the circuit. Thus, the plasma switch improves the rate at which thefuse blowout disconnects the electrical overstress from the circuit tobe protected. The resistive element, fuse and a plasma switch may all beformed in the same single wiring layer in a printed circuit board.

An example of a method of using an implantable medical device includesdetermining the condition of a patient with an implantable medicaldevice implanted, evaluating whether to initiate a course of shocktherapy, and delivering energy from an energy storage capacitor to thepatient through a circuit. While the shock is occurring detecting anypossible harmful electrical overstress condition in the shock therapyenergy, either harmful to the circuit or to the patient, and shuntingthe energy away from the functional circuit and the patient, to anenergy sink. There are several methods to perform the shunting,including shunting the shock energy to the energy sink using a groundelectrical connection, either by burning out a fuse between the shockcapacitor and the circuit, or by adding an electrical switch between theend of the fuse located nearest the shock therapy energy storagecapacitor and the ground electrical connection. The switch may be aplasma switch because plasma switches may most rapidly divert energyaway from the fuse being burned out and thus prevent more of thecapacitor overstress from reaching the circuit or the patient.

The detailed description refers to the accompanying drawings that show,by way of illustration, specific aspects and examples in which thepresent disclosed examples may be practiced. These examples aredescribed in sufficient detail to enable those skilled in the art topractice aspects of the present invention. Other examples may beutilized, and structural, logical, and electrical changes may be madewithout departing from the scope of the disclosed examples. The variousexamples are not necessarily mutually exclusive, as some examples can becombined with one or more other examples to form new examples.

Although specific examples have been illustrated and described herein,it will be appreciated by those of ordinary skill in the art that anyarrangement that is calculated to achieve the same purpose may besubstituted for the specific examples shown. This application isintended to cover any adaptations or variations of examples of thepresent invention. It is to be understood that the above description isintended to be illustrative, and not restrictive, and that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Combinations of the above examplesand other examples will be apparent to those of skill in the art uponstudying the above description. The scope of the present disclosedexamples includes any other applications in which examples of the abovestructures and fabrication methods are used. The detailed descriptionis, therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims, along with thefull scope of equivalents to which such claims are entitled.

What is claimed is:
 1. A method, comprising: delivering defibrillationshock energy from an implantable shock therapy energy storage capacitorto a subject through an electrical circuit; in response to the delivereddefibrillation shock energy exceeding a threshold, triggering a plasmaswitch to switch into shunting the defibrillation shock energy beingdelivered from the capacitor away from the electrical circuit and to acircuit power or ground node.
 2. The method of claim 1, furthercomprising shunting the capacitor shock therapy energy away from theelectrical circuit using at least one of a plurality of resistors, afuse, a transistor, a Hall effect switch, a magnetic switch, amicroswitch and a solenoid to open an electrical connection to theelectrical circuit and close an electrical connection to the groundnode.
 3. The method of claim 1, comprising shunting the capacitor shocktherapy energy away from the electrical circuit using a plasma switch toopen an electrical connection to the electrical circuit and close anelectrical connection to the ground node.
 4. The method of claim 1,further comprising selecting the threshold by selecting a fuse material,a fuse material vaporization temperature, a fuse cross sectional area, afuse length, and a fuse resistance to cause a fuse connected between thecapacitor and the electrical circuit to heat to the fuse vaporizationtemperature at the threshold, to vaporize the fuse and form an openconductor to the electrical circuit.
 5. The method of claim 4, furthercomprising forming a nonconductive gap between the fuse material and theelectrical connection to the ground node, the gap having a dimensionselected to be within a cloud of vaporized fuse material from thedelivered defibrillation shock energy exceeding the threshold andforming an electrical conduction path to the ground node with the cloud.6. The method of claim 1, comprising detecting an electrical overstresscondition in a the electrical circuit and in response to detecting theelectrical overstress condition, triggering the plasma switch.
 7. Amethod of using an implantable medical device, comprising: determining apresent condition of a patient having an implantable medical deviceimplanted within the patient; evaluating the present condition anddetermining whether to initiate a course of shock therapy; deliveringdefibrillation shock energy from an implantable shock therapy energystorage capacitor to a subject through an electrical circuit; inresponse to the delivered defibrillation shock energy exceeding athreshold, triggering a plasma switch to switch into shunting thedefibrillation shock energy being delivered from the capacitor away fromthe electrical circuit and to a circuit power or ground node.
 8. Themethod of using of claim 7, wherein shunting the shock therapy energy tothe energy sink includes shunting to the ground node.
 9. The method ofusing of claim 8, wherein shunting the shock therapy energy to theground node includes burning out a fuse located between the shocktherapy energy storage capacitor and the electrical circuit.
 10. Themethod of using of claim 9, comprising forming the plasma switch betweenan end of the fuse located nearest the shock therapy energy storagecapacitor and the ground electrical connection.
 11. The method of usingof claim 9, comprising forming the plasma switch from fuse vapor of thefuse.
 12. A method, comprising: forming an implantable medical devicecircuit on at least one of a printed wiring board, a hybrid circuitboard, and an integrated circuit device; forming a fuse between a powersource and the medical device circuit; and forming a plasma switchbetween the fuse and a ground source; wherein the plasma switch is for,in response to a delivered defibrillation shock energy exceeding athreshold, triggering a disconnecting the power source from the medicaldevice circuit and for triggering an at least temporary connecting ofthe power source to the ground source.
 13. The method of claim 12,wherein forming the plasma switch includes locating a portion of anelectrical conductor connected to the ground source close to at leastone of a portion of the fuse and a portion of a second electricalconductor connecting the power source and the fuse.
 14. The method ofclaim 13, wherein forming the plasma switch includes locating a portionof the electrical conductor connected to the ground source close to theportion of the fuse.
 15. The method of claim 13, comprising selecting atleast one of a distance between the electrical conductor connected tothe ground source and the portion of the fuse, a material of the fuse, athickness of the fuse, a length of the fuse, a width of the fuse, aresistance of the fuse and a power source voltage level to select thethreshold for triggering the plasma switch.
 16. The method of claim 15,comprising selecting the distance between the electrical conductorconnected to the ground source and the portion of the fuse to select thethreshold.
 17. The method of claim 15, comprising selecting a propertyof at least a portion of the fuse to vaporize as a cloud of ions inresponse to the delivered defibrillation shock energy exceeding athreshold.
 18. The method of claim 17, comprising selecting the distancebetween the electrical conductor connected to the ground source and thefuse to be substantially equal to a minimum design rule space for twounconnected metal features on a printed circuit board, a hybrid and anintegrated circuit.
 19. The method of claim 12, wherein forming animplantable medical device circuit on at least one of a printed wiringboard, a hybrid circuit board, and an integrated circuit device includesforming a hole in a dielectric of at least one of a printed wiringboard, a hybrid circuit board, and an integrated circuit device, withthe fuse disposed over the hole.
 20. The method of claim 12, comprisingforming the implantable medical device circuit on the printed wiringboard.